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Oxalaia
Oxalaia (in reference to the African deity Oxalá) is a genus of spinosaurid theropod dinosaur that lived in what is now the Northeast Region of Brazil during the Cenomanian of the Late Cretaceous, sometime between 100.5 to 93.9 million years ago. Its only known fossils were found in 1999 on Cajual Island in the rocks of the Alcântara Formation, which is known for its abundance of fragmentary, isolated fossil specimens. The remains of Oxalaia were described in 2011 by Brazilian paleontologist Alexander Kellner and colleagues, who assigned the specimens to a new genus containing one species, Oxalaia quilombensis. The species name refers to the Brazilian quilombo settlements. Oxalaia is the eighth officially named theropod species from Brazil and the largest carnivorous dinosaur discovered there. It is closely related to the African genus Spinosaurus. Although Oxalaia is known only from two partial skull bones, Kellner and colleagues found that its teeth and cranium had a few distinct features not seen in other spinosaurids or theropods, including two replacement teeth in each socket and a very sculptured secondary palate. Oxalaia's habitat was tropical, heavily forested, and surrounded by an arid landscape. This environment had a large variety of lifeforms also present in Middle-Cretaceous North Africa, due to the connection of South America and Africa as parts of the supercontinent Gondwana. As a spinosaurid, the traits of Oxalaia's skull and dentition indicate a largely piscivorous (fish eating) lifestyle similar to that of modern crocodilians. Fossil evidence suggests spinosaurids also occasionally preyed on other animals such as small dinosaurs and pterosaurs. Discovery and Naming Oxalaia stems from the Alcântara Formation, a succession of sedimentary rocks that is part of the Itapecuru Group of the São Luís Basin, in northeastern Brazil. These rocks have been dated by scientists to the Cenomanian stage of the Late Cretaceous, 100.5 to 93.9 million years ago.12 Outcropping at the northern coast of the formation, the Laje do Coringa locality is made up mostly of sandstones and mudstones, along with conglomerate rock layers containing fossil plant and vertebrate fragments.3 These sediments were deposited under marine and fluvial conditions similar to those of the Bahariya Formation in Egypt, where Spinosaurus remains have been found.14 In 1999, fossils of Oxalaia were recovered from the Laje do Coringa.5 Palaeontologist Elaine Machado, of the National Museum of Brazil, was surprised to find such a well-preserved fossil at the site and stated in a press release that "this is how most scientific discoveries happen, it was by accident".6 The finding was a rare occurrence due to the erosive nature of the tides at the deposit, which are responsible for the fragmented state of most fossils in the bone bed; remains not found on site are often removed from the formation by wave action.5 Generally, the majority of fossil remains found at the Alcântara Formation consist of teeth and isolated skeletal elements, of which the Laje do Coringa site has yielded hundreds. Oxalaia is one of three spinosaurids discovered in Brazil, the other two being Irritator and its possible synonym Angaturama, both of which were also initially known from partial skulls. They were discovered in the Romualdo member of the Santana Formation, part of the Araripe Basin. Microfossils date this layer member to the Albian, around nine to six million years before Oxalaia.589 The fossil record of spinosaurids is poor compared to those of other theropod groups; very few body fossils are known and most genera have been erected from isolated elements such as vertebra or teeth.1011 The holotype specimen of Oxalaia, designated MN 6117-V, was found in situ (at its original place of deposition) with part of the left side embedded in the rock matrix; it consists of the fused praemaxillae (front upper jaw bones) from a large individual. An isolated and incomplete left maxilla fragment (MN 6119-V) was referred to Oxalaia because it showed the same general traits occurring in spinosaurids, the maxilla was discovered on the rock surface, having possibly moved from its original location after erosion. Both bone fragments were found on Cajual Island, Maranhão, in northeastern Brazil, and are now housed at the National Museum of Brazil in Rio de Janeiro. Besides the partial skull bones, numerous spinosaurid teeth had earlier been reported from the Laje do Coringa site.5 The discoveries of Oxalaia and of the Late Cretaceous reptiles Pepesuchus and Brasiliguana were announced in a presentation by the Brazilian Academy of Sciences in March 2011.1213 Machado described Oxalaia as "the dominant reptile of is now Cajual Island". She stated that there is interest in spinosaurids in Brazil and abroad because of their debut in the Jurassic Park franchise and their distinctiveness among other carnivorous dinosaurs.12 The species description of Oxalaia was written by Brazilian palaeontologists Alexander Kellner, Elaine Machado, Sergio Azevedo, Deise Henriques, and Luciana Carvalho. This paper, among many others, were composed into a volume of 20 works on prehistoric biodiversity that was published by the Academy in March 2011.6 The type species Oxalaia quilombensis is the eighth officially named species of theropod from Brazil. The generic name Oxalaia is derived from the name of the African deity Oxalá, which was introduced into Brazil during the slavery period. The specific name quilombensis refers to the quilombo settlements like those on Cajual Island, which were founded by escaped slaves. Description The holotype praemaxilla is approximately 201 mm (7.9 in) long, 115 mm (4.5 in) wide (maximal estimated width is 126 mm (5.0 in)), and 103 mm (4.1 in) in height. Based on skeletal material from related spinosaurids, the skull of Oxalaia would have been an estimated 1.35 m (4 ft 5 in) long;5 this is smaller than Spinosaurus's skull, which was approximated at 1.75 m (5 ft 9 in) long by Dal Sasso and colleagues in 2005.14 Kellner and his team compared the Dal Sasso specimen (MSNM V4047) to Oxalaia's holotype snout in 2011; from this they estimated Oxalaia at 12 to 14 metres (39 to 46 ft) in length and 5 to 7 tonnes (5.5 to 7.7 short tons) in weight, making it the largest known theropod from Brazil,5 the second largest being Pycnonemosaurus, which was estimated at 8.9 m (29 ft) by one study.1215 The tip of the rostrum (snout) is enlarged and the rear-end constricted, forming the terminal rosette shape that distinguishes spinosaurids;5 this form would have interlocked with the also-expanded front lower jaw.16 The rostrum of Oxalaia features broad, deep holes that are possibly nutrient canals for blood vessels and nerves; it is also rounder in side view than that of Spinosaurus, whose upper jaw ends in a more acute downward angle as shown by specimens MSNM V4047 and MNHN SAM 124. The maxillae show a pair of elongated and thin processes extending forwards along the midline of the roof of the mouth; they are encased between the praemaxillae and border an elaborate, triangle-shaped pit at their front end. Similar processes are present in Suchomimus, Cristatusaurus, and MNHN SAM 124, although not as exposed.5 These structures compose the animal's secondary palate.517 The undersides of the praemaxillae are greatly ornamented in Oxalaia, in contrast to the smoother condition it has in other spinosaurids. The praemaxillae have seven alveoli (tooth sockets) on each side, the same number found in Angaturama, Cristatusaurus, Suchomimus, and MNHN SAM 124 (referred to Spinosaurus); MSNM V4047, another upper jaw specimen from Spinosaurus, had only six. It cannot be confirmed whether this lower number of teeth is due to ontogeny; for that, a larger sample size is necessary. A large diastema separates the third tooth socket from the fourth; this is observed in all other spinosaurids, being smaller in Suchomimus. Another diastema of nearly equal length is found between the fifth and sixth alveolus; this diastema is seen in MNHN SAM 124 and is much longer in MSNM V4047 but is absent from Suchomimus and Cristatusaurus. The maxilla fragment referred to Oxalaia (MN 6119-V) has two alveoli and a broken third one that includes a partial tooth. Like the praemaxilla, it had preserved nutrient canals. It also features a shallow dent in the middle, suggesting it was located near the external nares (nasal openings). Small fragments inside some of the remaining alveoli show that unlike its Early Cretaceous relatives Suchomimus and Cristatusaurus, Oxalaia lacked serrations on its teeth. Apart from the single, functional tooth in each socket, there were two replacement teeth,5 which according to Kellner are "a common feature in sharks or in some reptiles, but not in theropods".13 A cross-section of the teeth showed the typical oval shape exhibited by spinosaurs rather than the lateral compression of other theropod teeth. The spinosaurid teeth reported from Laje do Coringa were classified into two primaries morphotypes by palaeontologist Manuel Medeiros in 2006. Both show typical spinosaurine dentition, though morphotype II has smoother tooth enamel than the first.18 Oxalaia's teeth display a closer morphology to morphotype I while the second grouping of teeth represent either worn down morphotype I teeth or an undescribed spinosaurine from the Alcântara Formation. Classification Category:Dinosaurs of South America Category:Spinosaurids Category:Cretaceous dinosaurs